The present invention is directed to a fiber optic conduit-connector assembly for passing optical fibers between component housings. More specifically, this invention relates to a flexible, crush resistant and pressure tight conduit having at least one connector that is compatible with a standard RF coaxial cable entry port through which optical fiber pigtails are passed for connecting to other optical fibers or optoelectronic components.
Light wave communication over optical fibers has been effectively employed between the distribution headend and distribution nodes of broadband CATV systems. The distribution headend is the origination point of all signals carried on the system. The headend typically includes an antenna system, signal-processing equipment, combining networks and other related equipment. Television signals from commercial broadcast stations are collected from large "off air" television antennas and satellite earth receiving stations and transmitted to the signal processing equipment. The signal processor includes a number of electronic to the distribution network, filtering out unwanted signals, adjusting the output level or strength of the collected signals so that all signals that are carried have close to the same level, converting the collected signals to transmission channels that are optimized for application to the cable television system, and converting UHF signals to VHF cable channels. Finally, the combining network groups the signals for each cable channel into a single output for connection to the distribution network.
The portion of the distribution system which connects the headend output to the distribution nodes is generally called the trunk system and is designed for bulk transportation of multi-channeled broadband CATV signals throughout the area to be covered by the cable system. Typically, the trunk system consists of coaxial cable with a series or cascade of 20 or more trunk amplifiers installed at intervals along the coaxial cable. The trunk amplifiers are necessary to compensate for the inherent losses (attenuation) in signal strength resulting from transmission via coaxial cable. While trunk amplifiers are effective in compensating against signal loss, the amplifiers themselves introduce noise and distortion into the television signals carried by the system. In a CATV system where the subscriber area is large, the number of amplifiers required will be great, thereby increasing the amount of noise and distortion introduced into the system by these components.
The distribution nodes are the points at which signals from the trunk system are fed to a system of distribution lines that bring the signals to individual subscribers. Directional couplers and/or splitters are used to select a portion of the signal from a trunk amplifier to be fed to bridging amplifiers. Distribution or feeder lines are fed from each bridging amplifier to the subscriber areas, and signals are tapped from the feeder lines for connection to subscriber residences. The trunk cable continues to other distribution nodes, where the signals are routed through distribution lines to other subscribers.
Using fiber optic cable to replace the coaxial cable and trunk amplifier cascade forming the trunk system significantly reduces signal attenuation, noise and distortion in the CATV distribution system. Optical fibers have low signal attenuation in comparison to conventional coaxial cable. Thus, signals in the form of light may be transmitted for long distances over optical fibers without requiring amplification. Use of fiber optic transmission media eliminates the twenty or more trunk amplifiers needed in coaxial cable trunk systems and the noise and distortion caused by such amplifiers. The optical signals carried by the fiber optic trunk lines are converted to radio frequency (RF) electrical signals ("optoelectronic conversion") at the distribution nodes. The optoelectronic conversion provides broadband (e.g. 50-550 MHz) RF signals, which are then fed via bridging amplifiers to the feeder system of distribution lines to the individual subscribers.
In connecting optical fibers to the distribution nodes, a group of individual fibers is separated out from the bundle of fibers forming the fiber optic trunk cable. The separated fibers are then cut and connected to short lengths of optical fibers commonly referred to as pigtails. This connection may be accomplished, for example, by fusion splicing, which creates a permanent junction between the fiber ends. The splice connections are typically housed in splice enclosures, which serve to protect and organize the optical fiber splices. The opposite ends of the fiber optic pigtails are then inserted through an entry port in the housing for the optoelectronic conversion components for converting the optical signals to electrical RF signals.
Heretofore, fiber optic pigtails passing between a splice enclosure and an optoelectronic converter housing have been inserted through a plastic or vinyl tube. This tubing provided minimal protection for the fibers from moisture. The tubing was neither watertight nor pressure tight, nor did it protect the fibers from physical damage resulting from compressive or tensile forces. Also, this tubing decreased in flexibility at low temperatures. Additionally, the plastic or vinyl tubing did not prevent leakage of electromagnetic interference into the optoelectronic converter housing. This leakage could result in the introduction of noise into the converted electrical RF signals.
Accordingly, it is an object of this invention to provide a fiber optic conduit and connector that may be attached to a standard coaxial cable entry port.
It is another object of this invention to provide a fiber optic conduit and connector that is pressure tight and watertight.
It is still another object of this invention to provide a fiber optic conduit and connector that is shielded resistant and has high pull strength.
It is yet another object of this invention to provide a fiber optic conduit and connector that remains flexible throughout a wide range of operating temperatures.
It is a further object of this invention to provide a fiber optic conduit and connector that is shielded against leakage of electromagnetic interference.
These and other objects are achieved by the fiber optic conduit and connector of the present invention. In a preferred embodiment, the conduit has a highly flexible, crush resistant and fluid resistant inner tube formed of helically convoluted radiation-crosslinked, ethylene tetrafluoroethylene (ETFE). The inner tube is surrounded by a conductive material layer formed of braided stainless steel that provides both EMI shielding and tensile (pull) strength for the conduit-connector assembly. The outer layer of the conduit is formed of a moisture proof and pressure proof jacket formed of polypropylene that provides protection in an outdoor environment.
The connector is formed of a conductive, non-corrosive material, such as aluminum. In a preferred embodiment, the connector has a receptacle end for receiving the inner tube of the conduit that is internally threaded to threadingly engage the helical convolutions of the inner tube. The opposite end of the connector is adapted to engage a standard coaxial cable entry port.
The connector is permanently affixed to at least one end of the conduit by first stripping away the end portion of the outer jacket of the conduit, screwing the inner tube into the internally threaded receptacle end of the connector and wrapping the conductive shielding and pull strength layer around the outer surface of the connector receptacle end.
In one preferred embodiment, a thermofit ring is used to secure the connector to the conduit. Upon application of heat to the thermofit ring, the ring diameter shrinks, thereby crimping the shielding and pull strength layer to the outer surface of the connector end. A layer of heat shrink tubing is then placed over the thermofit ring and exposed inner layers of the conduit to provide a protective moisture proof and pressure proof layer.
In a second preferred embodiment, an externally threaded collet ring is placed over the shielding and pull strength layer and an internally threaded clamp nut is screwed onto the collet ring to secure the shielding and pull strength layer to the connector end. A layer of heat shrink tubing is then placed over the clamp nut and exposed inner layers of the conduit to provide a protective pressure proof layer. The inner diameters of the conduit inner tube and connector are sufficiently large to allow at least four optical fiber pigtails to pass through them.